1. Field of the Invention
The present invention relates to a method for producing trichlorosilane, and more specifically to a method for producing trichlorosilane from tetrachlorosilane in a more efficient manner.
2. Description of the Related Art
Trichlorosilane (SiHCl3: TCS) is a compound useful as a raw material for the production of highly pure polycrystalline silicon (also called polysilicon). Trichlorosilane reacts with hydrogen at a temperature of 1000° C. or higher to deposit high purity polysilicon. This deposition is mainly represented by the following reactions:4SiHCl3→Si+3SiCl4+2H2  (1)SiHCl3+H2→Si+3HCl  (2)
Trichlorosilane used for the deposition of polysilicon is generally produced by the reaction of metal silicon and hydrogen chloride. For example, Japanese Patent No. 3324922 discloses a method for producing trichlorosilane by reacting metal silicon with hydrogen chloride in the presence of a catalyst containing iron and aluminum in a fluidized bed reactor, as depicted in Reaction 3:Si+3HCl→SiHCl3+H2  (3)
According to the method, gases formed as a result of the reaction of the metal silicon and the hydrogen chloride are cooled to −10° C. or below, followed by condensation. The condensate includes other chlorosilanes as well as trichlorosilane. The trichlorosilane is isolated and collected from the condensate by distillation. The trichlorosilane is used as a raw material for the production of polysilicon. Tetrachlorosilane (SiCl4: STC) isolated by distillation is converted to trichlorosilane (TCS), as depicted in Reaction 4:3SiCl4+2H2+Si→4SiHCl3  (4)
That is, the tetrachlorosilane is reused for the production of polysilicon.
On the other hand, Japanese Patent Publication No. Sho 56-73617 suggests another method for producing trichlorosilane. According to the method, metal silicon particles having a size of about 100 to about 300 μm, hydrogen chloride, tetrachlorosilane, and hydrogen are supplied to a fluidized bed reactor filled with metal silicon particles. In the reaction vessel, the metal silicon reacts with the hydrogen chloride to form trichlorosilane (Reaction 3), and simultaneously, the metal silicon, the tetrachlorosilane, and the hydrogen react with each other to form trichlorosilane (Reaction 4). This fluidized bed process is illustrated in FIG. 1. As the reactions proceed, the metal silicon particles are gradually reduced in size, which requires the replenishment of fresh metal silicon particles. Since a change in the temperature of the raw materials is monitored to determine when to replenish the fresh metal silicon particles, the reaction temperature is not constant and fluctuates, and as a result, the quality of the product becomes non-uniform depending on the reaction time.
Under these circumstances, there is a need for a method by which chlorosilanes, particularly tetrachlorosilane, present in gases released during the production of polysilicon from trichlorosilane can be more efficiently converted to trichlorosilane and can thus be reused for the production of polysilicon.